NGC 0257. Left panel: outline of the spiral arms on the deprojected SDSS g-band map. Middle panel: colour map of the oxygen abundance distribution of the spiral arms (opaque dots) and the interarm region (transparent dots). Right panel: radial density distribution in the oxygen abundance-galactocentric distance space of the spaxels located in the spiral arms (green contours) and those associated with the interarm region (orange colour map). The outermost contour encircles 100% of the total number of spaxels, decreasing by 20% in each consecutive contour. The lines represent the error-weighted linear fit derived for the arm (green dashed line) and interarm (orange solid line) distributions. The inset indicates the number of spaxels of each distribution that contribute to the derivation of the gradient, i.e. located within the radial range 0.5–2.0 r_e.